Current-detection coil for a current transformer

ABSTRACT

A current-detection coil for a current transformer operating by the Rogowski principle has an insulating annular body made of a fiber-reinforced thermosetting plastic and manufactured as a single-piece solid body in a divisible mold. The parting line of a mold serving for manufacturing the annular body is in a flattened region on the outer periphery of the annular body and on a flattened region on the inner periphery. The annular body allows current transformers operating by the Rogowski principle to be manufactured that are capable of measuring in the entire range of currents that may occur and are resistant to mechanical and thermal stresses.

FIELD OF THE INVENTION

The present invention is directed to a current-detection coil for acurrent transformer operating according to the Rogowski principle.

BACKGROUND INFORMATION

A previously proposed current-detection coil is discussed in the BritishEPE Journal, vol. 3, No. 1, March 1993, pp. 51-59, where simple solidtoroids or flexible cut-open coil bodies are proposed as annular bodyshapes. Furthermore, the use of circular cut-open plastic rings with arectangular cross section, which can be bent for placement on aconductor and then return to their annular shape, is discussed therein.Each of the various annular bodies used in the current-detection coilsdiscussed in the EPE Journal are used to form a Rogowski currenttransformer.

Although all of the current-detection coil embodiments of the EPEJournal can be made to exhibit good transfer characteristics, when theyare mass-produced, however, virtually each unit will exhibit differenttransfer characteristics and therefore individual adjustments orelectronic correction circuits must be used to achieve uniform results.The effect of small deviations of the geometry of Rogowski currenttransformer coils from the ideal shape on their transfer characteristicsis known. One solution proposed is to use polished straight ceramic rodsas winding supports. See German Journal Technische MitteilungenAEG-Telefunken 60 (1970) 7, pp. 444-448. Because, however, acurrent-detection coil operating according to the Rogowski principlemust surround a conductor on a closed path, the use of straight ceramicrods requires that a closed current-detection coil be made of fourpartial coils. In this case, the rectangular junction of the componentparts causes considerable interference in the electromagnetic field. Inorder to remove this influence, the junction points are made field-freethrough the use of a shielding. The desired accuracy of the Rogowskicurrent transformer is therefore associated with considerable expensefor the manufacture of the current-detection coils.

SUMMARY OF THE INVENTION

The present invention is directed to a current-detection coil with anelectrically insulating annular body as a support for a winding, theshape of the annular body cross section having flattened regions on theouter periphery and on the inner periphery.

An object of the present invention is to provide a current-detectioncoil of a Rogowski current transformer that exhibits a minimum spread ofits characteristics and is suitable for both regular current measurementand triggering.

This object is achieved according to the present invention by designingthe remaining regions of the cross section shape in an approximatelysemicircular shape. The annular body is made from a fiber-reinforcedthermosetting plastic material as a solid single-piece body in adivisible mold, with the annular body being made without burrs andhaving no undercuts, and the winding is applied with a toroidal corewinding machine.

The annular body provided according to the present invention is a verystable component having high heat resistance and a low thermal expansionfactor. These characteristics result in minimum changes in the windingshape when, for example, the current-detection coil is applied to themain current paths of a low-voltage power switch, which can exhibitstrong mechanical shocks and temperatures up to more than 100° C. Theflattened cross section shape on the inner and outer periphery of theannular body contributes to this stability of the annular body byincreasing the resisting moment against mechanical deformation. Theburr-free manufacture of the annular body contributes to the desireduniformity and quality of the winding by the fact that the winding wireis in tight contact with the entire periphery of the annular body andtherefore cannot be displaced or deformed.

An annular body having an outer diameter of approximately 80 to 100 mm,and wound with a single layer of lacquered copper wire that isapproximately 0.05 to 0.1 mm thick is particularly well suited forapplication in low-voltage power switches in the medium-to-high ratedcurrent ranges.

If current-detection coils according to the present invention are usedin multipole low-voltage power switches, the measurement accuracy in theadjacent phases can be influenced by the small distances between therespective conductors. This influence depends on the quality of thecurrent-detection coil winding. According to the present invention, theinterference of the adjacent conductors can be limited by dimensioningthe infeed of the toroidal core winding machine so that a gap of max.0.5% of the periphery of the annular body remains between the beginningand end of the winding for a given number of spires.

Burr-free manufacturing of the current-detection coil annular body canbe facilitated according to the present invention by using a device inwhich the parting line of the mold is arranged in the flattened regionof the cross-section shape of the annular body. Subsequent operations toremove any remaining burrs are therefore either unnecessary or can belimited to minimum effort.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of an annular body of a current-detection coilaccording to the present invention.

FIG. 2 shows a cross-section view of the annular support body.

FIG. 3 shows a portion of an injection mold used for manufacturing theannular body according to the present invention.

FIG. 4 shows a cross-section view of a current-detection coilarrangement.

FIG. 5 shows a top view of a current-detection coil arrangement.

DETAILED DESCRIPTION

FIGS. 1 and 2 show an annular body used as a support of a winding for acurrent transformer operating according to the Rogowski principle.Annular body 1 has a cross section shape that is flattened on an innerand outer periphery thereof, with the remaining profile being of anapproximately semicircular shape. FIG. 3 shows a mold 2 that is used tomanufacture annular body 1. In FIG. 3, the parting line 3 is located inthe outer flattened region 4 and the inner flattened region 5 of thecross section shape of the annular body 1. The plastic injectioncompound that comprises annular body 1 is preferably afiberglass-reinforced polyester thermosetting resin mix, which providesannular body 1 with good dimensional stability, resistance againstdeformation, and low thermal expansion under the temperatures occurringin operation.

The beginning and the end of winding 8, which is illustrated in FIG. 1,is applied to annular body 1 by using a toroidal core winding machine.As can be seen in FIG. 1, there is a very small gap 9 between thebeginning and the end of the winding. This gap 9 is achieved for thegiven number of spires by a constant and suitably dimensioned infeed ofthe toroidal core winding machine used for the manufacture of thebody 1. Since the distance of the windings is uniform along the entireannular body 1, and since gap 9 is very small in relation to theperiphery of annular body 1, faults such as may occur due to magneticfields of adjacent phases displacing one another in a low-voltage powerswitch have a minimum effect. As a result, the finishedcurrent-detection coil of the present invention is capable of handling acurrent range starting at a few amperes (A) and extending toapproximately 150 kA. In particular, the lower measuring limit is notonly determined by the characteristics of the current-detection coil,but also by the characteristics of an electronic analyzer circuit, towhich the current-detection coil is connected. Therefore currentsstarting at approximately 50 A can be reliably measured.

Due to the position of parting line 3 in regions 4 and 5 of annular body1 (FIG. 2), there is little tendency for burrs to form duringmanufacture. If, however, burrs are formed, they may be removed in asubsequent operation prior to applying the winding to annular body 1with the toroidal core winding machine. If the entire range of currentsthat may occur is to be measured with a single winding, the winding musthave a large number of spires. Therefore, a coil wire having a verysmall diameter must be used. An annular body with an outer diameter of100 mm and a cross section of approximately 8×12 mm, for example, canhave a winding with approximately three thousand spires using a 0.07 mmthick lacquered copper wire.

A finished current-detection coil 6 can be arranged as shown in FIGS. 4and 5. The arrangement shown there has a housing 7 open on one side,which has a central window 10 for the passage of a rail-shaped conductorof a power switch. Current-detection coil 6 manufactured using anannular body 1 according to FIGS. 1 and 2 is in an annular space 11 ofhousing 7 and is fastened and covered with a sealing compound 12. Thesame sealing compound 12 serves for fastening an additional inductivecurrent transformer 13, which is omitted from FIG. 5. Currenttransformer 13 can be used, for example, for providing auxiliary powerfor an electronic analyzer circuit to be connected to current-detectioncoil 6. Furthermore, an electronic triggering mechanism can be operatedusing current transformer 13.

What is claimed is:
 1. A current-detection coil for use with a currenttransformer operating in accordance with the Rogowski principle,comprising:an electrically insulating annular body; and a windingapplied to the electrically insulating annular body,wherein: across-section shape of the electrically insulating annular body includesa first flattened region located at an outer periphery of theelectrically insulating annular body and a second flattened regionlocated at an inner periphery of the electrically insulating annularbody, the outer periphery and the inner periphery of the electricallyinsulating annular body are joined by approximately semicircularregions, the electrically insulating annular body comprises afiber-reinforced thermosetting plastic formed as a single-piece solidbody, the electrically insulating annular body is burr-free withoutundercuts, and the winding is applied to the electrically insulatingannular body using a toroidal core winding process.
 2. Thecurrent-detection coil according to claim 1, wherein the electricallyinsulating annular body is formed using a divisible mold having aparting line which is arranged on the first and second flattenedregions.
 3. A current-detection coil for use with a current transformeroperating in accordance with the Rogowski principle, comprising:anelectrically insulating annular body; and a winding applied to theelectrically insulating annular body,wherein: a cross-section shape ofthe electrically insulating annular body includes a first flattenedregion located at an outer periphery of the electrically insulatingannular body and a second flattened region located at an inner peripheryof the electrically insulating annular body, the outer periphery and theinner periphery of the electrically insulating annular body are joinedby approximately semicircular regions, the electrically insulatingannular body comprises a fiber-reinforced thermosetting plastic formedas a single-piece solid body, the electrically insulating annular bodyis burr-free without undercuts, the winding is applied to theelectrically insulating annular body using a toroidal core windingprocess, and the electrically insulating annular body has an outerdiameter of approximately 80 to 100 mm and is tightly wound with asingle layer of lacquered copper wire having a diameter of approximately0.05 to 0.1 mm.
 4. A current-detection coil for use with a currenttransformer operating in accordance with the Rogowski principle,comprising:an electrically insulating annular body; and a windingapplied to the electrically insulating annular body,wherein: across-section shape of the electrically insulating annular body includesa first flattened region located at an outer periphery of theelectrically insulating annular body and a second flattened regionlocated at an inner periphery of the electrically insulating annularbody, the outer periphery and the inner periphery of the electricallyinsulating annular body are joined by approximately semicircularregions, the electrically insulating annular body comprises afiber-reinforced thermosetting plastic formed as a single-piece solidbody, the electrically insulating annular body is burr-free withoutundercuts, the winding is applied to the electrically insulating annularbody using a toroidal core winding process, and a gap between abeginning portion and an end portion of the winding has a width of nomore than approximately 0.5% of the periphery of the electricallyinsulating annular body.